The present invention relates to a solid state image sensor comprising a semiconductor body, an opaque bottom electrode applied on a main surface of the semiconductor body, a photoelectric converting film applied on the bottom electrode, a transparent surface electrode applied on the photoelectric converting film and a read-out transistor provided in the semiconductor body and having an active region connected to the bottom electrode for reading-out charge carriers induced in said photoelectric converting film in response to an incidence of light input.
The known solid state image sensors can be roughly classified into a MOS type image sensor and a charge transfer type image sensor depending upon the construction of the signal reading-out means. Today, the high integration density and large scale array has been developed and practical solid state image sensors have been manufactured on a commercial basis. In accordance with the high integration density of the solid state image sensor, the ratio of an aperture area to an area of a photoelectric converting section has become smaller and smaller, and as a result of this, signal detection with high sensitivity has become difficult to attain.
In order to increase the aperture ratio of the solid state image sensor there has been recently proposed a solid state image sensor having a so-called layered-construction, in which the photoelectric converting section is provided on the signal reading-out switch or the signal reading-out transfer section. An example of such a solid state image sensor is described in "Technical Report of Japan Television Society", Vol. 3, No. 33, 1980, January, pp. 41 to 46. A typical example of such a solid state image sensor is shown in FIG. 1.
In FIG. 1, the solid state image sensor comprises a P type substrate 1 and an N channel signal reading-out MOS transistor switch 2 whose source diffusion region 3 is connected to a bottom electrode 5 of a photoconductive film 4. Light input is made incident upon the photoconductive film 4 via a transparent electrode 6 and electron-hole pairs are induced therein. The electrons are absorbed into the transparent electrode 6 and the holes flow into the bottom electrode 5 to increase the potential of the electrode 5, whose potential has been initially set at a low value, e.g. 0 V. It can be considered that the electrons stored in the initial set condition disappear due to their recombination with holes that are conducted into the bottom electrode 5. Then the reading-out can be effected by making the MOS transistor switch 2 conductive to supplement the electrons that have disappeared by means of drain electrode 7, drain diffusion region 8, and the N channel source region 3. That is to say, an electric current due to the supplemental electrons can be derived as a photoelectrically converted output signal.
The most favorable property of this construction is that the bottom electrode 5 can be extended almost to a boundary between adjacent image sensor cells and therefore, the aperture ratio can be made substantially equal to 1. Moreover, the following properties can be obtained.
(1) Since the spectrum sensitivities of the photoconductive films are different from each other in dependence upon their composition, the most preferable photoconductive film can be selected in accordance with a desired characteristics of the image pick-up device to be constructed.
(2) Since the potential of the bottom electrode does not exceed the potential of the transparent electrode even under the strong light input, so-called "blooming" can be inhibited.
(3) Short wavelength components of the incident light input which could not be detected by normal photodetectors, such as a photodiode, can be effectively converted in the surface region of the photoconductive film in the vicinity of the transparent electrode and thus the photoelectric conversion efficiency can be improved.
As explained above, the solid state image sensor of layered-construction has several advantages as compared with other known solid state image sensors, except for possible problems in manufacture. However, when one wishes to increase the integration density of the solid state image sensor and to decrease the size of each of the picture element cells, it will become much more difficult to detect a small signal without decreasing the signal-to-noise ratio. Further, in the case of effecting the image sensing under weak incident light input such as a high shutter speed image sensing, the signal-to-noise ratio is small and the image quality is deteriorated to a great extent. Such problems have been considered to be inevitable both in the solid state image sensor in which the charge carriers induced by photoelectric conversion are supplemented and in the solid state image sensor in which the charge carriers are directly transferred.